1 Answer
1

I don't understand how this is a UX question or how this name helps you, but I will explain the functionality anyway.

A crosshair is a pair of perpendicular lines (horizontal and vertical) that move when the mouse is moved or hover some important points on the graphic (depending on the requirements/ implementation). Usually, there are also two labels displayed on the X- and Y-axes in the points where the axes are crossed by the crosshair lines or a tooltip with the data.

Sometimes, a line or a label of the crosshair is hidden - the example from your image.

Maybe this example helps you more (it is more clear the functionality with a crosshair on both axes):

Crosshairs are thin vertical and horizontal lines centered on a data point in a chart. When you, as a chart creator, enable crosshairs in your charts, your users will then be able to target a single element

"I don't understand how this is a UX question or how this name helps you" Perhaps OP wants to refer to it in tool-tips / help-text / documentation.
– TripeHoundJul 24 at 12:28

@TripeHound A developer will know what to do, how to implement it or what it is. Never a designer sent me this kind of notes with names for every line (I'm also a front-end dev). This is why I added in my answer details about the functionality, that can be useful for UX. The name, not so important, nobody cares.
– Madalina TainaJul 24 at 13:23

Without wanting to get bogged-down in a minor detail of a good answer... I never mentioned "developer". I'm talking about a term the user might see either directly in the UI or other documentation. If there is an established term for a particular UI feature (and "cross-hairs" would seem to be the right one here), then using that name can be important and is, I would argue, part of UX/UI (although I am predominately a developer, so not an expert on such matters).
– TripeHoundJul 24 at 13:40

@TripeHound I understand your point of view. I still don't think the name could help the user or the designer...
– Madalina TainaJul 24 at 13:44

How about if someone’s patenting an idea where every single thing needs to be properly described?
– essdeepeeJul 26 at 13:21